Transmitter for code communication systems



Patented Oct. 11, 1949 UNITED STATES PATENT OFFICE TRANSMITTER FOR CODECOIVIIHUNICATION SYSTEMS Application April 8, 1948, Serial No. 19,655

Claims.

In systems of the type commonly described as carrier currentcommunication systems, various frequency bands or channels for speechand for code signaling are provided, over which independentcommunication may be established without interference.

It is an object of my invention to provide for use in systems of thischaracter, an improved code transmitter which is particularly adapted totransmit code pulses of alternating current of difierent frequencieswithin the voice frequency range, in a reliable manner.

Another object of my invention is to provide a plurality ofnon-interfering code transmitters suitable for use in carrier currentsystems of the type in which a voice frequency or speech channel issubdivided to provide a plurality of closely spaced code communicationchannels, each of which includes receiving apparatus selectivelyresponsive to energy of a different frequency.

Another object of my invention is to provide an improved alternatingcurrent code transmitter in which the alternating current circuits, withthe exception of a single frequency determining unit, are non-reactive,enabling similar transmitters to be employed for generating any one of aplurality of different frequencies by the insertion of a removable unitadjusted to the desired frequency.

A further object of my invention is the provision of improved means forgraduating the rise and fall of the alternating currents constitutingthe codepulses delivered by the different transmitters so as to minimizethe transient components having frequencies different from the assignedfrequency, thereby decreasing the effect upon adjacent channels andenabling the channels to be more closely spaced. This is accomplished bythe provision of novel means for causing the output current to rise andfall gradually to form the code pulses, which possesses an advantageover transmitters employing resonant filters for that purpose in thatthe output frequency is maintained constant during the periods ofchangin amplitude and transient components of the output current due tofilter reaction are avoided.

A feature of my invention is the provision of an improved oscillatorwhich is particularly adapted to generate alternating currents of asubstantially pure sine wave form, free from harmonics, so that resonantfilters are not needed in the output circuits of the differenttransmitters for the suppression of such harmonics.

A further object of my invention is the provision of an oscillator forgenerating voice frequency current which is stabilized so as to delivercurrent of constant voltage and frequency regardless of ordinaryvariations in the supply voltage or temperature.

Other objects of my invention and features of novelty therein will beapparent from the following description taken in connection with theaccompanying drawing.

I shall describeone form of transmitter embodying my invention and shallthen point out the novel features thereof in claims.

In practicing my invention, I provide a vacuum tube oscillator of thephase shift type, the output of which is "supplied to a second vacuumtube arranged in the manner of a cathode follower and a portion of theoutput of the cathode follower is supplied as feed back energy to theinput circuit of the phase shift oscillator. Another portion of theoutput of the cathode follower is supplied to a third vacuum tube,constituting a bufler amplifier, the output of which is supplied throughan output transformer to the communication channel. The oscillatoroperates to generate alternating current continuously as long as poweris supplied thereto, while the buffer amplifier serves as a keyingamplifier and supplies amplified alternating current to the outputtransformer ,only when the contact of a transmitting relay occupies oneof its two positions. When the transmitter relay contact is in the otherof its two positions, it completes a circuit by which a positivepotential is applied to the cathode of the keying amplifier tube whichis sufiicient to render the tube non-conducting. A resistance-capacitynetwork associated with the keying amplifier cathode circuit causes thistube to change gradually from its conducting to its non-conducting statewhen the positive biasin voltage is applied by the closing of thetransmitting relay contact, and similarly causes the keying amplifier tochange gradually from its non-conducting to its conducting state whenthe positive biasing voltage is cut off by the opening of thetransmitting relay contact, the frequency of the alternating currentoutput remaining constant during both these periods.

The single accompanying drawing is a diagrammatic view showing apreferred embodiment of my invention.

Referring to the drawing, the communication channel to which coded voicefrequency currents are supplied by the apparatus of my inventioncomp-rises the line wires LI and L2, extending between two spacedlocations designated by the reference characters A and ,B. Thecommunication channel is here shown as comprising a physical pair ofwires supplied with energy from the transmitter output terminals Ti andT2, but it is to be understood that the communication fier, the outputof which is supplied to the grid [3 of the cathode follower tube VTZ. Asexplained below, the energy supplied through the frequency determiningunit FDU to the input circuit of tube VTI is of the proper phase tomaintain the oscillations.

I'he vacuum tube VTZ is connected in the well known manner of a cathodefollower, which differs from a conventional amplifier in that thechannel may be extended to include Ia speech channel, for example,constituting the input channel of a carrier current system or radiolink,

or it may be one of the speechc'hannels of a.

multi-channel carrier current communication system.

At location A a transmitting relay T is shown, the winding of which issupplied with energy by a circuit, not shown, so that the contacts ofrelay T are operated in accordance with the code signals to betransmitted from location A to location B. At location B there isprovided a receiving relay R, the contacts of which are to be operatedin step with the code pulses transmitted from location A. The circuitsfor controlling relay T and those controlled by the contacts of relay Rform no part of my invention, and may be conventional telegraphcircuits, or circuits for a code type of remote control system, forexample, various types of which are well known. As shown, the contactsof relays T and R are normally biased to their right-hand position.

The transmitter comprises an oscillator tube VTI having associatedtherewith a frequency determining unit FDU, a cathode follower tube VTZ,a buffer or keying amplifier tube VT3, and an output transformer OT,together with the necessary resistors and condensers associated with thevacuum tubes. High voltage direct current energy is supplied from asuitable source, not shown, the positive terminal of which is designatedby the reference character B(+) and the negative terminal of which isgrounded. The circuits for supplying energy to the heaters of the tubeshave not been shown in order to clarify the drawing.

The oscillator tube VTI may (be of any suitable type but as here shownis a pentode, having control circuits of the phase shift type. The

' control grid 5 of tube V'Il is supplied with energy 1 by tube VTZthrough a phase shifting network, or frequency determining unit FDU,comprising condensers C1, C2, C3 and C4, and resistors RI, R2, R3 andR4. The cathode l of tube VTI is provided with a cathode bias resistorR5 bypassed by-condenser C5 to afford a ground connection ofrelativelylow impedance to the alternating current components of the tube current.A suppressorgrid 8 is connected to the cathode, as in a conventionalpentode amplifier circuit, and a screen grid 9 is supplied with apositive potential of a suitable value by means of a voltage dividercomprising resistors RB and R1 connected between terminals B(+) andground. A by-pass condenser Cl effectively grounds the screen gridinsofar as the alternating current component of the tube current isconcerned.

The plate 1 l of tube VTl is supplied with high voltage direct currentfrom the terminal B(+) of the power supply through resistor R9, and isconnected through a coupling condenser C9 to the grid 83 of vacuum tubeVT2.

From the foregoing it will be apparent that the tube VT! and itsassociated condensers and resistors comprises a single stage pentodeamplioutput is taken from the cathode circuit rather than from the platecircuit, and a portion thereof is applied to the input circuit toprovide a negative feedback for stabilizing purposes. The plate I5 isdirectly conected to terminal B(+) and the cathode I! is connected toground through resistors RIO and R6 I. A grid resistor RI2 is connectedat the junction point is of resistors RIO and R! l.

It will be clear from a consideration of the circuits that the potentialapplied to grid l3 due to the drop across resistor RI?! is decreased bythe drop across resistor RH) in the output circuit of tube VT2 toprovide the negative feedback referred to.

The output voltage of tube VT? as measured across resistors Rl'fi and RHis nearly equal to the input voltage supplied .by tube VII, and differsbut slightly in phase due to the drop across the coupling condenser C9which preferably is of relatively large capacity. This output voltage isimpressed on the frequency determining unit by reason of the connectionfrom condenser C! over wire 25 to the cathode I1 and from the resistorsR! to R4 and RH to ground. Each condenser CI to C4 in the unit is inseries with an associated resistor Rl to R4 and is adapted to cause aphase shift which varies in accordance with the impressed frequency. Ata particular frequency the total shift in phase is degrees as requiredto maintain the oscillations in tube VT! and it will be apparenttherefore that the tube will oscillate at a frequency determined by thconstants of the frequency determining unit. It will also be apparentthat since the required phase shift is not obtained for frequencieswhich are multiples of the predetermined frequency, harmonics will besuppressed and the current delivered by tube VTI will be of asubstantially pure sine wave form.

It will also be apparent that the frequency at which the circuitoscillates may be adjusted by proper selection of the values of thecondensers and resistors in the frequency determining unit, since theresponse of the output circuits of tubes VTI and VTZ is independent ofthe frequency. In practice the frequency determining unit FDU isconstructed as a self-contained removable unit,

I so that the frequency of the transmitter may be readily changed byusing different frequency determining units therein. The resistor R4 maybe of the adjustable type, to permit a variation in the resistancethereof to be made, in order to compensate for variation in thecomponents due to manufacturing tolerances, and thereby enable each unitto be more accurately tuned to its specified frequency.

The use of a cathode follower rather than a conventional amplifierfollowing the oscillator stage is advantageous from the standpoint offrequency response, because although the cathode follower is inherentlydegenerative, and has an amplification factor less than unity, it iscapable of providing power amplification, and is essentiallynon-inductive so that for different frequencies of oscillation theenergy level of the output of the cathode follower remains substantiallythe same. Additionally, its characteristics are such that changes in theplate resistance-of the cathode follower tube and variations in the loadresistance with temperature as occur in practice do not cause sufiicientchange in the oscillating circuit to cause detrimental variations in thefrequency or output energy level.

Since the normal bias yoltage for the grid of the vacuum tube VTIconstitutes the drop across the cathode resistor R5, it will be apparentthat 'a sudden change in the voltage of the direct current energysupplied to the transmitter will cause 'a corresponding change in thenormal bias voltage. This would interfere with operation if suflicientto cause the bias voltage to go beyond the cut-off value of the tubeVTI, and accordingly, to prevent this occurrence, tube VTI is arrangedto have a remote cut-off, that is, the value of grid Voltage at whichthe tube remains conducting is in excess of that reached due to ordinaryvariations in the high voltage power supply.

From the foregoing, it will be seen that the oscillator is in continuousoperation and the energy supplied therefrom to the cathode followertubeappears as a voltage across the cathode resistors RIB and RI I. Aportion of the output voltage of tube VT2 is supplied over an adjustableconnection on resistor RI I, to the grid circuit of the amplifier tubeVT3. This grid circuit comprises a coupling condenser CH and a gridresistor RI3 of relatively high value, connected to the grid 23 ofvacuum tube VT3 in the usual manner. The plate 25 of vacuum tube VT3 isconnected to the positive terminal B(+) of the high voltage power supplythrough a primary winding 21 of I the output transformer OT which has asecondary winding 29 connected to the output terminals TI and T2 of thetransmitter.

The cathode 3I of tube VT3' is connected to ground through a biasingresistor Rl5 and bypass condenser CI5 in multiple, and is supplied ,witha positive biasing voltage from the high voltage power supply by acircuit including aresistor RH and contact 32 of the transmitting relayT,,whenever this contact is closed.

By this means, the cathode 3| of vacuumtube VT3 ismaintained at arelatively high positive potential when contact 32 of relay T is in itsnormal position, as shown. As a result, the tube VT3 is normallynon-conducting. When relay T is' energized and opens contact 32, thesupply of bias voltage to the cathode 3I is interrupted. I-Iowever, thecondenser CI5 and resistor R are selected so thata definite timeinterval is required until the condenser CI5, which previously had beencharged through resistor RI I, discharges sufficiently to cause tube VT3to .become conductive. It will also be apparent that the rate of changein the direct current supplied to tube VTB will be graduated further dueto the fact that this is supplied through an inductive reactancecomprising the primary winding 21 of transformer OT. When tube VT3 isconducting, it acts as a conventional amplifier, so that the voicefrequency output of the oscillator and cathode follower is amplified andsupplied to the output terminals Ti and T2 through the outputtransformer OT.

Considering now the location B, at which the voice frequency alternatingcurrent supplied over the line wires LI and L2 by the transmitter atlocation A is received. The band-pass filter BPF at, location B is tunedto the frequency of- -the ;transmitter just described. and accordinglyselece.

oscillator to the grid circuit of tube VT3.

tively passes the energy supplied over the line wires from terminals TIand T2 to the detector amplifier DA, where the voice frequency energy isamplified and rectified and then supplied to the winding of relay B, sothat the contact 35 of relay R is operated between its normal andreverse positions substantially in unison with the contact 32 of thecorresponding transmitter relay T.

It will be understood that each time relay T :clos'es its contact 32, itreestablishes the circuit previously described for supplying a positivebias voltage to the cathode 3| of vacuum tube VT3. However, since thecondenser CI5 is substantially discharged at this time, the voltageacross the condenser will rise at a relatively slow rate when energy issupplied thereto over the circuit including resistor RI! and contact 32of relay T. Accordingly, the voltage of cathode 3i will rise at acorrespondingly slow rate so that the plate current of tube VT3 is notcut off abruptly when contact 32 closes, but decreases gradually at LaFrom the foregoing, it will be seen that the operation of contact 32 ofrelay T causes pulses of voice frequency energy to be supplied from thetransmitter at location A over the line wires LI and L2 to location B,where the Pulses are selectively filtered by the band-pass filter BPFand thus amplified and detected by'the detector amplifier DA, andsupplied to the winding of relay B, so that the contacts of relay Raccurately repeat the code operation of the contacts of relay T.

r As indicated by the output transformer OT, 3 other voice frequencytransmitters similar to the one described but tuned totdifferentfrequencies may be connected to the line wires LI and L2,

' the additional frequencies being selectively filteredat'the receivinlocation by sharply tuned band-pass filters BPF, etc., similar to filterBPF, .each selectively responsive to a different one of a plurality offrequencies provided by different frequency determining units FDU.

;. The output transformer CT is arranged so that it presents. arelatively high impedance to the :currentssupplied to the line wires LIand L2 fjby other transmitters, thereby minimizing the loss of energywhen the number of transmitters is relatively large.

As previously explained, the band-passfilters are preferably sharplytuned, and the number of available channels is increased by allotting varelatively narrow band to each channel. If the I transmitter circuits.were arranged to cause a sharp rise and fall in the voice frequencypulses,

as would. be the case, for example, if condenser viii-were omitted, thecorrespondingly abrupt changes in the currents supplied to the receivingfilters would lessen their discriminatin characteristics and possiblylead to the false operanels. This tion of relays R associated withadjoining chandifiiculty remains, to a lesser degree, when resonantcircuitelements and filters are interv,posed in the transmitter outputcircuit to reduce ;th e rate of change of the currents constitutingamazon the code pulsesfor the'reascn that such devices inherentlyproduce transient currents in response to changes in'amp'litude of theimpressed. current, having frequencies Which differ from their steadystate resonant frequency. It will be clear that the transmitter of myinvention is adapted to produce a minimum of transient disturbance inthe receiving filters of adjacent channels, during the transmission ofcode' pulses, so that the discrimination of the filters with respect topulses constituting code signals is nearly the same as for steady stateconditions, and that consequently the code channels may be more closelyspaced. This is particularly desirable when the available frequenciesare limited by the transmission characteristics of a carrier currenttelephone systent, for example, as those within a band of frequencies ofthe minimum width acceptable for speech transmission.

Although I have herein shown and described.

only one form of voice frequency transmitter for a code communicationsystem, it is to be understood that various changes and modificationsmay be made therein within the scope of the appended claims withoutdeparting from the spirit and scope of my invention.

Having thus described my invention, what I claim is:

1. In combination, an electron tube amplifier having a cathode, a gridand a plate, an output circuit for said tube extending from the plate ofsaid tube through an impedance element to the positive terminal of adirect current source and from the cathode of said tube through abiasing resistor and condenser in parallel to the negative terminal ofsaid source, an input circuit for said tube comprising a source ofalternating current of substantially constant amplitude and frequencyconnected between the grid of said tube and said negative terminal, andkeying means for at times causing said tube to-"be conductive to supplyamplified alternating current to said outcapacitance network connectedbetween said cathode and the negative terminal of said sourcetodecreasethe rate of rise and fall of said biasing voltage due to theoperation of said transmitting contact whereby there is supplied to saidoutput terminals code pulses of alternating current each pulse of whichbuilds up and dies down according to the rise and taller said biasingvoltage.

3. A transmitter for use in a code communication system employing codepulses of alternating current, comprising a source of alternatingcurrent of substantially constant amplitude and frequency, an amplifiertubehaving a control grid, an anode, and a cathode, means forcontinuously applying a positive potential to said anode from a directcurrent source-and an alternating current potential to said control gridfrom said source ofv alternating current, keying means for producingsaid pulses comprising a transmitting contact operable toa first and asecond position at a given code rate for applying a positive biasingpoten tial to the cathode of said amplifier tube at its first positionwhereby said tube is rendered nonconductive and at its second positionremoving said biasing potential so that said tube is renderedconductive, a resistance-capacitance network associated with the cathodeof said tube for delaying the biasing and unbiasing of said tube, and anoutput transformer having a primary winding supplied with energy fromthe anode of said tube.

4. In a code transmitter for supplying code pulses of alternatingcurrent to a communication channel, the combination comprising a sourceof alternating current of substantially constant amplitude andfrequency, an output transformer having a secondary winding connected tosaid communication channel, and keying means for supplying code pulsesof alternating current from said source to a primary winding of saidoutput transformer, said keying means comprising an put circuit and atother times causing said tube ing so proportioned that the tube isrendered non conducting when said positive potential is supplied to thecathode and is rendered conducting when said connection is interrupted,said biasing resistor and condenser having a selected time constant forpredetermining the rise and fall of the voltage of the direct currentsupplied to the cathode of said tube in response to the periodicoperation of said transmitting contact.

2. A code transmitter comprising, in combination, a source ofalternating current of substantially constant predetermined amplitudeand frequency, an amplifier tube having a plate to cathode circuitconnected to a source of direct current and a grid circuit supplied withan alternating current potential from said source of alternatingcurrent, an output transformer having a primary winding in said platecircuit and a secondary winding connected to the output terminals ofsaid transmitter, and keying means for said transmitter comprising atransmitting relay adapted for code operation at a rate less than saidpredetermined frequency, means including a contact of "sale transmittingrelay for supplying a positive parser tube and means comprising aresistance amplifier tube having a plate circuit which includes theprimary winding of said output transformer, a grid circuit connected tosaid source of alternating current, a transmitting contact which in oneposition connects the cathode of said tube to a source of direct currentto thereby render said tube non-conductive, and a resistance capacitynetwork for predetermining the rate of change of potential of saidcathode in response to the periodic operation of said keying contact.

5; In a code transmitter for supplying code pulses of alternatingcurrent to a communication channel, the combination comprising, a sourceof alternating current of a given amplitude and frequency; an amplifiertube having a plate, a cathode and a control grid; a transformer havinga primary and a secondary winding, said plate connected to the positiveterminal of a source of direct current through said primary winding,said secondary winding connected to said channel, said grid connected toone terminal of said source of alternating current, a biasing circuitincluding a resistor and a capacitor in multiple, said cathode connectedto the negative terminal of said direct current source and to the otherterminal of said alternating current source through said biasingcircuit, a contact adapted for operation to a first and second positionat a given code rate, means including said first position of saidcontact and another resistor to connect said cathode to the positiveterminal of said direct current source for alternately rendering saidtube non-conductive and conductive at said code rate to supply codepulses of said alternating current to said channel,

9 10 and said biasing circuit proportioned for a time constant thatprovides a given rate at which said UNITED STATES PATENTS amplifier tubechanges from its conductive to its Number Name Date non-conductivecondition and vice versa. 21 Lea Sept. 1, 1925 DAVID P. FITZSIMMONS. 5 9Car er July 5, 1927 1,849,870 Fitzgerald Mar. 15, 1932 REFERENCES CITED2,141,343 Campbell Dec. 2'7, 1938 2,307,771 Denton Jan. 12, 1943 Thefollowlng references are of record in the 2,420,093 Place May 6, 1947file of ms patent 10 2,446,106 Robertson July 27, 1943

